With the extensive use of personal computers and other data processing facilities both at home and in the office, there are great needs for improved data communications. Hence, packet-switched public networks are being rapidly developed and increasingly interconnected with each other. These existing packet networks have mostly been serving data communications traffics as opposed to voice telephony.
Voice and data traffic have significantly different characteristics. Voice is typically continuous in one direction for relatively long intervals and tolerant of noise, but sensitive to variations in delay. Data is bursty and sensitive to noise errors, but tolerant of moderate delays and variations in arrival times.
Two fundamental different switching techniques have therefore been traditionally applied to voice and data transmissions. Circuit switching, where switched connections between users are dedicated for call duration, is the basis of the present-day switched voice telecommunication network. On the other hand, packet switching, where data packets from multiple terminals share a single, high-speed line and are switched based on logical channel numbers attached in the packets, is being rapidly adopted as the basis of the present-day switched data telecommunication network.
Packet switching was pioneered in the ARPANET network of the U.S. Department of Defense, and has been widely implemented in a variety of public data networks. However, most public telephone systems are fundamentally circuit switched, which is an inherently inefficient system because typically each subscriber utilizes the allotted channel for a relatively small amount of the total connection time. Furthermore, the number of simultaneous circuit-switched communications are limited because only a portion of the available bandwidth is allocated to such communications.
Another disadvantage is that, because circuit switching is centralized, a failure at the switching center can result in failure of the entire network. A further disadvantage of circuit-switched telephony is due to the proprietary nature of the telephony switches currently in use. Because the switching software is often proprietary and not shared with other manufacturers, the cost and delay in adding and interfacing new services are often frustrating and installation prohibiting.
It has been proposed that packet-switched techniques replace, or at least be combined with some, circuit-switched telephony so that the entire system bandwidth may be made available to each subscriber on a random access basis. For this purpose, there are currently emerging software products that make use of the Internet, which is a constantly changing collection of interconnected packet-switched networks, for telephony. VOCALTEC software provides half-duplexed long-distance telephone capability through the Internet. Camelot Corp is another entry in the Internet telephone business with a MOSAIC front end software that supports full-duplexed voice conversation. These products offer an alternative to long-distance analog telephone service for the subscribers by digitizing and compressing voice signals for transport over the Internet.
Some limitations of this type of hybrid telephone system are: (1) Both the caller and the callee must have computers, (2) they must have sound systems on their computers, (3) they must have full Internet access, (4) they must have both purchased compatible software, (5) they must both connect to the Internet at the time the call is made, and (6) the telephony software must be in execution at both ends at the same moment. These limitations translate into a considerable amount of investment in hardware and software, which has to be made by the individual subscribers to implement such a telephony system. The last limitation also means that the calls have to be scheduled in advance in most cases, which clearly does not provide the convenience of conventional telephone calls. An additional problem with such software products is that the performance is constrained by the capabilities of each computer, such as processor speed, memory capacity, and modem functional features.